Fuel cells are electrochemical devices for converting chemical energy stored in fuels directly into electrical energy by performing an electrochemical reaction. In most cases oxygen or oxygen ions react with hydrogen, CO or other fuels, thereby generating a flow of electrons and consequently providing an electric current as well as heat.
The reaction employs a reducing agent and an oxidant as reactants, which are to be continuously fed to the fuel cell, typically the hydrogen is used as a reducing agent and oxygen or air containing such oxygen is used as an oxidant.
In most cases, a fuel cell can be used reversely to perform an electrolysis reaction, where an electrical current and possibly also heat have to be provided. For the sake of simplicity, only the fuel cell operation mode is described below.
A fuel cell power system in general comprises the following components: one or several fuel cell stacks, as well as auxiliary equipment also referred to as balance of plant. The fuel cell stack is made of individual repeating-units, which are modularly combined and electrically connected. The individual repeating-units contain one or several cell membranes, in which the electrochemical reactions as mentioned above, take place. The repeating-units contain also components to feed the reactants, allowing electrical contacting or sealing, etc.
The auxiliary equipment provides the conditioning of the feed streams, thus providing air or oxygen and the fuel at the correct temperature and pressure conditions as well as an optional fuel processor or fuel reformer. Furthermore the auxiliary equipment may include heat exchangers for the correct operating temperature of the fuel cell stack and for making use of the thermal energy generated by the electrochemical reactions to preheat fuel or oxidant feed streams, and to deliver useful heat to the user. An example for such a heat exchanger is disclosed in WO2006/048429 A1.
The auxiliary equipment may also include electrical energy management systems.
A cell membrane usually consists of an electrolyte in contact with an anode and a cathode on either side thereof. The electrolyte is an ionic conductor, but electric insulator. In operation as fuel cell, a fuel is fed continuously to the anode, thus the negative electrode and an oxidant is fed continuously to the cathode, thus the positive electrode. The electrochemical reactions take place at the electrodes to produce an ionic current through the electrolyte as soon as an electric current is allowed to flow from/to the respective electrodes through an external circuit, hence allowing performing a work on a load.
The unit cells comprising the cell membranes as mentioned above can have different shapes, such as plates or tubular structures. Each cell membrane has to be contacted electrically. In addition, the reactant gases have to be properly distributed over the surface of the electrodes to maximize the efficiency of the reaction. This is achieved for instance by creating gas distribution layers of specific geometry in contact with the surface of the electrodes. Both the electrical conduction and gas distribution are therefore often combined in specific parts. Together with the cell membranes and additional individual components, this sub-assembly represents one repeating-unit of the fuel cell stack.
For planar cell membranes, the individual repeating-units are most often placed on top of each other to form a stack.
In this case, in the repeating-units the gas distribution layers are used not only to transport the reactants to the electrodes, but also to conduct the electrical current from one electrode of a first cell membrane to the second electrode of another cell membrane, thereby connecting several cells in series.
In a unit cell, the dense electrolyte provides a physical barrier to prevent the fuel and oxidant gas streams from mixing directly. In planar stacks, bipolar plates usually ensure the same separation of gases between adjacent repeating-units, providing also the electrical contacting through the gas distribution layers.
A large number of catalyst sites are to be provided at the interfaces between the electrolyte layer and the electrodes, thus a zone which has mixed conductivity for electrons and ions. The performance of the fuel cell membranes has been continuously improved by efforts to increase the conductivity of the electrolyte, developing improved electrode catalytic activities and reactant transport, and broadening the temperature range over which the cells can be operated.
The electrodes are typically porous and are made of an electrically and possibly also ionically conductive material. At low temperatures, only a few relatively rare and expensive materials provide sufficient electro-catalytic activity, thus in these cases catalysts are deposited in small quantities at the interface between the porous electrode and the electrolyte. In high temperature fuel cells, a larger number of materials qualify for an electrode material thanks to their improved electro-catalytic activity.
The porous electrodes thus have the primary function of providing a surface for the electrochemical reactions to take place. In addition, their function is to conduct electrons away from or into the three-phase interface and provide current collection and connection with either other cells or the load.
While the performance of the cell membranes is principally dictated by the choice of materials, their size or microstructure and the way they are combined together, the performance of a fuel cell stack depends to a very important extent also on the quality of the distribution of reactants over the cell membranes, the electrical contacting of the electrodes, and the homogeneity of reactant flows and of temperatures among the different repeating-units. Last but not least, the choice of the fuel processing and of the operating points has an important impact on the performance and the lifetime of the fuel cell.
A variety of fuel cells has been developed and is currently under various stages of commercialization. The most common classification of fuel cells relates to the type of electrolyte used, such as solid oxide fuel cells (SOFC), polymer electrolyte fuel cells (PEFC), alkaline fuel cells (AFC), phosphoric acid fuel cells (PAFC) or molten carbonate fuel cells (MCFC).
A polymer electrolyte fuel cell (PEFC) has an electrode which is configured as an ion exchange membrane, in particular a fluorinated sulfonic acid polymer, which has the characteristic of being a good proton conductor. The only liquid present in the fuel cell is water, as the fuel is mostly a hydrocarbon fuel providing the hydrogen ions and the oxidant is air providing the oxygen for performing the electrochemical reaction. The operation temperature is usually less than 100° C. as the membrane must be hydrated by water and such water should therefore not evaporate faster than it is formed. Thus preferably the operating temperature is around 60° C. to 80° C. Typically carbon electrodes with a platinum electro-catalyst are used both for the anode and the cathode. The bipolar or separator plates are either made of carbon or metal. The fuel should not contain any CO as the anode is easily poisoned by traces of CO. An important commercial application for PEFC is fuel cell vehicles, as well as electrolyzers.
An alkaline fuel cell (AFC) has a KOH electrolyte, which is retained in a matrix, e.g. made of asbestos and a wide range of electro-catalysts can be used, e.g. Ni, Ag, metal oxides, spinels, noble metals. It is OH− ions that are the charge carriers across the electrolyte.
The operation temperature is usually about 250° C. if a KOH of a concentration of about 85 weight % is used and may be lower than 120° C. if a KOH of a concentration of 35% to 50% is used. The fuel may not contain any CO nor any CO2, which would react with the electrolyte to K2CO3, thereby altering it. Thus preferably pure hydrogen is used as a fuel for an AFC. Typically electrodes composed of transition metals are used with a platinum electro-catalyst are used both for the anode and the cathode; the bipolar plates are made of metal.
A phosphoric acid fuel cell (PAFC) uses highly concentrated phosphoric acid as the electrolyte which is retained in a matrix, e.g. made of silicon carbide and mostly platinum is used as an electro-catalysts. The ions transported in the electrolyte are protons. The typical operating temperature of a PAFC lies between 150° C. and 220° C. due to the fact that the concentrated phosphoric acid has a high stability even under these comparatively high temperatures. At lower temperatures, phosphoric acid is a poor ionic conductor and CO poisoning of the platinum electro-catalyst occurs. At the higher operating temperatures a content of up to 1% of CO as diluent is acceptable. Typically electrodes composed of carbon are used both for the anode and the cathode; the bipolar plates are made of graphite. Due to the corrosive nature of phosphoric acid, expensive materials such as graphite have to be used. The main field of use of PAFC is stationary applications.
A molten carbonate fuel cell (MCFC) uses a combination of alkali carbonates as the electrolyte, which is retained in a matrix of LiAlO2. The typical operating temperature of a MCFC is about 600° C. and 700° C. where the alkali carbonates form a highly conductive molten salt, with carbonate ions providing ionic conduction. The anode usually consists of nickel and the cathode of nickel oxide, the interconnects are made of stainless steel or nickel. The nickel/nickel oxide electrodes provide sufficient activity at the high operating temperature, thus an electro-catalyst is not needed. The fuel can comprise CO and hydrocarbons; furthermore a source of CO2 is required at the cathode, which can be provided by the exhaust from the anode. The main field of use of MCFC is stationary applications.
A solid oxide fuel cell (SOFC) uses a solid electrolyte, which is a non-porous metal oxide, such as 3%-10% yttria-stabilized zirconia (YSZ) that is ZrO2 stabilized by Y2O3, or Sm2O3-doped CeO2 (SDC) or GdO2-doped CeO2 (GDC). The typical operating temperature of a SOFC depends on the electrolyte material and is about 500° C. up to 1100° C. with oxygen ions providing ionic conduction. The anode and the cathode usually include also ceramic materials. The fuel electrode is usually made of a combination of metal and a ceramic forming a cermet, e.g. mostly Ni-YSZ cermets. The oxygen electrode usually comprises an electrically conductive doped perovskite or a combination of a perovskite and an ionic conductive ceramic such as YSZ or GDC. Typical perovskites used as cathode contain a combination of La, Sr, Co, Fe, Mn.
The bipolar plates are usually made of stainless steel.
Further information on possible components for cathode, anode and electrolyte as well as optional intermediate layers and catalysts can be found in U.S. Pat. No. 7,632,586 B2 incorporated by reference.
The fuel can comprise next to hydrogen CO and other hydrocarbons, such as methane or ammonia, whereas only H2 and CO are easily converted electrochemically. The other fuels are consumed indirectly or require a dissociation step before being converted. Furthermore, a SOFC can tolerate a fuel that is diluted by inert gases such as N2, CO2 or steam. Amongst the hydrocarbons, it can be natural gas, gasoline, diesel or also biogas. This type of fuel cell remains however sensitive to some poisoning elements contained in the fuels, such as sulphur, in particular H2S and COS that are considered as a poison already in a concentration of above 1 ppm.
The cathode-anode-electrolyte unit of the cell membrane is constructed with two porous electrodes that sandwich the electrolyte. Air flows along the cathode, thus transporting oxygen molecules to the cathode. When an oxygen molecule contacts the cathode/electrolyte interface it acquires electrons from the cathode. The oxygen ions diffuse into the electrolyte material and migrate to the other side of the cell where they contact the anode. The oxygen ions encounter the fuel at the anode/electrolyte interface and react catalytically, whereby water, carbon dioxide, heat and electrons are produced. The electrons are fed into the external circuit for providing electrical energy.
The main field of use of SOFC is stationary applications, such as stationary power generation, mobile power, auxiliary power for vehicles, specialty applications. The power densities usually attained by SOFCs are in the range of 200 to 500 mW/cm2 for stationary applications.
The SOFC is the fuel cell having undergone the longest continuous development period, starting in the late 1950's. Due to the fact that a solid electrolyte is foreseen, the cell membrane can be formed into a variety of shapes, such as tubular, planar or monolithic shapes. The electrical efficiencies depend largely on the used fuel. Using hydrogen as fuel, electrical efficiencies in the range of 45%-55% (LHV) can be achieved, with maxima close to 60% at the level of a repeating-unit. Using methane as fuel, system electrical efficiencies of 60% can be attained for stack electrical efficiencies close to 70%. Furthermore the emissions of acid gas or any solids are negligible.
An arrangement of a solid oxide fuel cell system for generating electric power by combination of oxygen with a reactive gas, i.e. a fuel gas is disclosed in WO2006/048429. The solid oxide fuel cell includes a stack configuration comprising an electrolyte layer sandwiched between two electrodes. One of the electrodes is in operation in contact with oxygen or air, the other electrode is in contact with a fuel gas at an operating temperature of about 500° C. to about 1100° C. Usually a support layer is used during the production of the cell to contain the electrode layer and to provide additional mechanical stability of the cells. The support layer may also function as a current collector.
The cathode comprises a perovskite, a lanthanum or strontium manganite or an yttria stabilized zirconia. Oxygen ions are formed from the oxygen gas provided at the cathode, which migrate through the electrolyte layer to combine with the hydrogen gas provided at the anode. The anode comprises nickel and/or yttria stabilized zirconia. At the anode, water is formed and electrons are provided, which are collected in the current collector.
One characteristic of fuel cell systems is that their efficiency is nearly unaffected by size. This means, that small, relatively high efficient power plants can be developed starting from a few kW for domestic cogeneration units to low MW capacity power plants.
A problem associated with fuel cells in general is the fact that a single cell membrane does generate a DC potential in the order of 1V, which is too small to be used for residential or automotive applications. For this reason, a plurality of cell membranes is combined to a stack of cell membranes connected electrically in series as to provide a voltage of sufficient magnitude to be converted efficiently to AC current and employed in most commercial applications.
Usual stacks are made of a few tens to a few hundreds of cell membranes connected partly in series and in parallel, with some designs including even a few thousands of cells.
The assembly of a stack of repeat-units should therefore at one hand require as few assembly steps as possible and on the other hand guarantee proper operating conditions for each of the cell membranes.
Due to the connection of repeat-units in series, any performance limitation on one single cell membrane may have important consequences on the overall performance of the stack, as it can limit the overall current that can be driven and therefore the resulting electrical power.
The stack construction depends on the type of cell membranes that are used. The first main class of stacks uses tubular cell membranes such as presented in WO01/91218 A2.
The second class of stacks uses planar cell membranes that can be interconnected by piling up. Among them, principal differences concern the type and geometry of fuel and oxidant supply, or the design of gas distribution over the electrodes and their electrical contacting.
A first concept which has been proposed e.g. in EP 1 864 347 B1 is a stack of cylindrical shape. Thus the cell membrane is a disk-shaped ceramic three layer membrane consisting of a positive electrode, an electrolyte and a negative electrode (CAE unit). The fuel is supplied in a central channel and directed radially outwardly and an oxygen containing gas is supplied from the periphery and directed toward the central channel.
In US2011/0269048A1, a stack concept based on rectangular cell membranes is shown, where said membranes are attached to a gas distribution unit presenting fuel inlet and outlet ports, and where the oxidant is supplied and extracted at the periphery of said gas distribution unit. In order to improve gas distribution of the gas flowing across the surface of the cell membrane the gas channels are curved. Previously, the tubular manifolds at the gas entry and exit section of the cell membrane have presented an obstacle to gas flow, which has resulted in an inhomogeneous flow field of the gas flowing across the cell membrane. According to US2011/0269048A1 curved gas channels are suggested, which guide the gas around the obstacles to the regions behind the obstacles. Thereby a more even distribution of gas flow can be obtained and the negative impact of the obstacles on gas flow be compensated.
The reactant supply and discharge of the solution presented in EP 1 864 347 B1 require according to U.S. Pat. No. 7,632,586 B2 a relative complicated manufacturing procedure for the interconnecting plates. To avoid this, the planar CAE units are positioned one above the other with interconnecting layers formed as planar metal plates arranged in between neighboring CAE units. The respective passages for fuel and oxidant are formed in the anode and cathode layers.
Furthermore the effects of expansion of the CAE unit and the structures for supplying the CAE unit with the reactants and conducting the reactants away therefrom have to be taken into account.
Moreover, the electrodes and interfaces tend to degrade as soon as excessive temperatures are reached.
Due to the exothermic reaction, an active cooling of the unit cells is therefore required, which can be principally achieved by air cooling. To limit temperature gradients and excessive temperature differences in the CAE unit and in the gas distribution structures, a proper distribution of the cooling air in the unit cell is required. To limit temperature differences, a large excess of cooling air is required with respect to the amount that would be necessary for the electrochemical reaction itself. This excess air implies additional losses in the balance of plant, in particular due to the consumption of the air blowers. These losses can however be reduced if the pressure drop in the stack is low, that means, if the gas distribution structure for the air in the stack presents a low resistance to the air flow.
An additional drawback of the use of excess air is the transport of poisoning species onto the air electrode. Especially volatile chromium is known to be released by the metallic components situated upstream of the stack and transported into the stack by the air stream. The volatile chromium tends to deposit in the air electrodes by electrochemical and chemical reactions. In particular, volatile chromium reacts spontaneously with the strontium contained in the electrodes. Moreover, it can be deposited electrochemically as chromium oxide at the electrode/electrode interface, hence reducing the number of reacting sites. Not only chromium, but also silicon, sulfur and other species are known to further affect the durability of the air electrode.
A problem associated with fuel cell stacks of the prior art is local temperature peaks developing on the surface of an electrode, which usually forms a planar layer.
If such local temperature peaks occur, the reaction kinetics may be altered and a local hot spot may be formed. Such a hot spot is undesired because it involves a high strain on the materials, by causing a local thermal expansion, which may lead to warpage or deformations of the layer materials affected. Due to the fact that the ceramics materials of the electrodes or the electrolyte are brittle, they may be subject to cracks and eventually break if subjected to substantial local temperature variations.
The occurrence of such hotspot can be drastically reduced by increasing the cooling air flow, and by proper design of the air distribution structure that contacts the CAE unit and hence can serve as heat dissipating structure.
The effect of thermal strain can further be mitigated in principle by a stack having a similar configuration as shown in U.S. Pat. No. 6,670,068 B1. Thus a plurality of CAE units are in electrically conductive contact with a contact plate and a fluid guiding element is formed as shaped sheet metal part and connected to the contact plate in a fluid-tight manner by welding or soldering. Thereby the contact plate defines a fluid chamber having a combustible gas or an oxidizing agent flowing through it during operation of the fuel cell unit. The shaped sheet metal part is disposed with a plurality of corrugations giving it a wave-like structure. The wave-like structure as such may compensate for some of the thermal expansion of the CAE unit and of the fluid guiding element in operation. However due to the local contact of the wave peaks or wave troughs with the respective electrode, the fluid guiding element has to follow the thermal expansion of the electrode. If the fluid guiding element does not have sufficient elasticity the strain due to thermal expansion is introduced into the electrode. The electrodes are formed from solid, brittle ceramics. Thus, if a high strain is introduced into the electrodes, cracks may be formed, which will ultimately destroy the electrode. In addition the welding or soldering connection provided between the fluid guiding element and the anode also contributes to the stiffness of the construction. In particular if materials having a different coefficient of thermal expansions are used, the strains may finally lead to damages of the electrode and may damage the cell membrane concerned. In particular the flow of reactants may be altered or direct mixing of them can occur if the cell membrane is broken, leading to spontaneous combustion. Thus locally hot spots may form, which may induce local thermal expansion and thus further development of local stress.
An additional solution for mitigating the effects of thermal strain and thermal expansion is provided in WO2004/021488. This solution foresees a frame of a first and a second foil-like element enclosing a fuel passage. A CAE unit is attached to the first of the foil-like elements with the anode being arranged immediately adjacent to the first foil like element on the opposite side of the fuel passage. The fuel reaches the anode by traversing the first foil-like element, which is disposed with perforations for this purpose. The second foil like element is fluid-tight and serves as a separating element to separate fuel flow from the flow of the oxide containing gas, such as air. A good electrical contact is ensured by providing a wire mesh in the fuel passage and by providing a further wire mesh on the second foil like element on the side opposite of the fuel passage. The supporting structure of WO2004/021488 can thus expand quite freely, and the close bonding of the CAE unit to the foil-like elements plays a role of a heat dissipating structure.
Thus it is an object of the invention to improve existing fuel cells, to make them more reliable, and to allow cheaper manufacturing.